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Review the sorting problems fixed in v4.3.1 and update the code to:
* Make it clearer to the reader how it works.
* Better at choosing the number of threads to use.
* Optimise the data per thread (uses less memory in some cases).
* Add optional debug output.
* Remove the real cause of the bug in v3.4 (affected only when all
  the data fitted into RAM with a single thread which is why it only
  appeared when the default RAM size was increased).